Re: [PATCH v3 4/6] powerpc: Introduce temporary mm

["Christopher M. Riedl" <cmr@codefail.de>]

On Thu Aug 27, 2020 at 11:15 AM CDT, Jann Horn wrote:
> On Thu, Aug 27, 2020 at 7:24 AM Christopher M. Riedl <cmr@codefail.de>
> wrote:
> > x86 supports the notion of a temporary mm which restricts access to
> > temporary PTEs to a single CPU. A temporary mm is useful for situations
> > where a CPU needs to perform sensitive operations (such as patching a
> > STRICT_KERNEL_RWX kernel) requiring temporary mappings without exposing
> > said mappings to other CPUs. A side benefit is that other CPU TLBs do
> > not need to be flushed when the temporary mm is torn down.
> >
> > Mappings in the temporary mm can be set in the userspace portion of the
> > address-space.
> [...]
> > diff --git a/arch/powerpc/lib/code-patching.c b/arch/powerpc/lib/code-patching.c
> [...]
> > @@ -44,6 +45,70 @@ int raw_patch_instruction(struct ppc_inst *addr, struct ppc_inst instr)
> >  }
> >
> >  #ifdef CONFIG_STRICT_KERNEL_RWX
> > +
> > +struct temp_mm {
> > +       struct mm_struct *temp;
> > +       struct mm_struct *prev;
> > +       bool is_kernel_thread;
> > +       struct arch_hw_breakpoint brk[HBP_NUM_MAX];
> > +};
> > +
> > +static inline void init_temp_mm(struct temp_mm *temp_mm, struct mm_struct *mm)
> > +{
> > +       temp_mm->temp = mm;
> > +       temp_mm->prev = NULL;
> > +       temp_mm->is_kernel_thread = false;
> > +       memset(&temp_mm->brk, 0, sizeof(temp_mm->brk));
> > +}
> > +
> > +static inline void use_temporary_mm(struct temp_mm *temp_mm)
> > +{
> > +       lockdep_assert_irqs_disabled();
> > +
> > +       temp_mm->is_kernel_thread = current->mm == NULL;
>
> (That's a somewhat misleading variable name - kernel threads can have
> a non-NULL ->mm, too.)
>

Oh I didn't know that, in that case yes this is not a good name. I am
considering some changes (based on your comments about current->mm
below) which would make this variable superfluous.

> > +       if (temp_mm->is_kernel_thread)
> > +               temp_mm->prev = current->active_mm;
> > +       else
> > +               temp_mm->prev = current->mm;
>
> Why the branch? Shouldn't current->active_mm work in both cases?
>
>

Yes you are correct.

> > +       /*
> > +        * Hash requires a non-NULL current->mm to allocate a userspace address
> > +        * when handling a page fault. Does not appear to hurt in Radix either.
> > +        */
> > +       current->mm = temp_mm->temp;
>
> This looks dangerous to me. There are various places that attempt to
> find all userspace tasks that use a given mm by iterating through all
> tasks on the system and comparing each task's ->mm pointer to
> current's. Things like current_is_single_threaded() as part of various
> security checks, mm_update_next_owner(), zap_threads(), and so on. So
> if this is reachable from userspace task context (which I think it
> is?), I don't think we're allowed to switch out the ->mm pointer here.
>
>

Thanks for pointing this out! I took a step back and looked at this
again in more detail. The only reason for reassigning the ->mm pointer
is that when patching we need to hash the page and allocate an SLB 
entry w/ the hash MMU. That codepath includes a check to ensure that
->mm is not NULL. Overwriting ->mm temporarily and restoring it is
pretty crappy in retrospect. I _think_ a better approach is to just call
the hashing and allocate SLB functions from `map_patch` directly - this
both removes the need to overwrite ->mm (since the functions take an mm
parameter) and it avoids taking two exceptions when doing the actual
patching.

This works fine on Power9 and a Power8 at least but needs some testing
on PPC32 before I can send a v4.

> > +       switch_mm_irqs_off(NULL, temp_mm->temp, current);
>
> switch_mm_irqs_off() calls switch_mmu_context(), which in the nohash
> implementation increments next->context.active and decrements
> prev->context.active if prev is non-NULL, right? So this would
> increase temp_mm->temp->context.active...
>
> > +       if (ppc_breakpoint_available()) {
> > +               struct arch_hw_breakpoint null_brk = {0};
> > +               int i = 0;
> > +
> > +               for (; i < nr_wp_slots(); ++i) {
> > +                       __get_breakpoint(i, &temp_mm->brk[i]);
> > +                       if (temp_mm->brk[i].type != 0)
> > +                               __set_breakpoint(i, &null_brk);
> > +               }
> > +       }
> > +}
> > +
> > +static inline void unuse_temporary_mm(struct temp_mm *temp_mm)
> > +{
> > +       lockdep_assert_irqs_disabled();
> > +
> > +       if (temp_mm->is_kernel_thread)
> > +               current->mm = NULL;
> > +       else
> > +               current->mm = temp_mm->prev;
> > +       switch_mm_irqs_off(NULL, temp_mm->prev, current);
>
> ... whereas this would increase temp_mm->prev->context.active. As far
> as I can tell, that'll mean that both the original mm and the patching
> mm will have their .active counts permanently too high after
> use_temporary_mm()+unuse_temporary_mm()?
>

Yes you are correct. Hmm, I can't immediately recall why prev=NULL here,
and I can't find anything in the various powerpc
switch_mm_irqs_off/switch_mmu_context implementations that would break
by setting prev=actual previous mm here. I will fix this for v4. Thanks!

> > +       if (ppc_breakpoint_available()) {
> > +               int i = 0;
> > +
> > +               for (; i < nr_wp_slots(); ++i)
> > +                       if (temp_mm->brk[i].type != 0)
> > +                               __set_breakpoint(i, &temp_mm->brk[i]);
> > +       }
> > +}